Method for accurately controlling the volume of ink droplets emitted from a print head

ABSTRACT

A method for controlling printing actions of a print head ( 1 ) comprising pumps ( 10 ) filled with ink ( 18 ), and actuators ( 16 ) for generating actuation pulses acting on the ink( 18 ), comprises the step of determining a characteristic frequency of the pumps ( 10 ). As the characteristic frequency of the pumps ( 10 ) is directly related to the geometry of the pumps ( 10 ), the characteristic frequency can be used as an indicator of the state of the pumps ( 10 ) and the volume of the ink droplets emitted by the pumps ( 10 ). In case a slight change of the characteristic frequency is detected, the actuation pulse is adjusted in order to still meet the requirements regarding the volume of the ink droplets. In case a relatively large change of the characteristic frequency is detected, the printing action of the pump ( 10 ) concerned is stopped, and may be taken over by another pump ( 10 ).

The present invention relates to a method for controlling the volume ofdroplets of printing fluid emitted from a print head during a printingprocess, said print head comprising: at least one pump having an inletfor taking in the printing fluid, a pump chamber for containing theprinting fluid and an outlet for letting out the printing fluid; and anactuator for generating actuation pulses acting on the printing fluid inthe pump.

Printing is a well-known technique for laying down a layer on a carrierconsisting of paper, glass, plastic or another suitable material ormixture of materials. A type of printing technique in which the layer isformed by spraying a printing fluid on the carrier is commonly referredto as ink-jet printing technique.

For the purpose of carrying out the ink-jet printing technique, ink-jetprinters have been developed. These printers comprise a print head inwhich a large number of miniature valveless pumps are integrated. Eachpump is associated with an actuator for influencing the pressure of theprinting fluid in the pump. When the actuator is actuated, the pressurein the pump is increased, as a result of which the pump delivers exactlyone droplet of printing fluid, the droplet having a specified flightdirection, speed and size. As the actuators may be controlledindividually, it is possible to exactly determine when a pump needs tofire a droplet and when the same pump needs to retain the printing fluidon the basis of the characteristics of a desired printed pattern.

The concept of firing and retaining droplets of printing fluid accordingto a predetermined schedule is often referred to as drop-on-demand(DOD). The technology of DOD print heads has developed along two mainlines, which has resulted in two main types of print heads.

A first main type of print head is a bubble-jet print head. In abubble-jet print head, each pump contains a small heating element thatis in direct contact with the printing fluid. When a droplet needs to beemitted, the heating element is switched on, as a result of which theprinting fluid in contact with the heating element is quickly heated toa relatively high temperature. During the process, the heat flux is sohigh that during the switch-on time the heat penetrates only a thinfluid layer, causing a vapour bubble to grow almost explosively at apredetermined spot in the pump. This growing vapour bubble causes asmall amount of liquid to be pushed at a high velocity through theoutlet of the pump.

A second main type of print head is a piezo-jet print head. In apiezo-jet print head, each pump has its own piezo-electric actuator.When charged, the actuator deforms, causing a pressure rise in the pumpthat leads to droplet emission.

The present invention will be described in the context of piezo-jetprinting, which does not imply that the present invention is notapplicable to bubble-jet printing as well.

DOD ink-jet printing proves to be an enabling technology for themanufacturing of displays comprising a large number of light emittingdiodes, which displays are commonly referred to as PolyLED displays.Each light emitting diode (commonly referred to as LED) comprises astack of individual layers. A number of these layers is formed by dosingthe material of these layers dissolved in a solvent into a pixel, apixel being a limited area having predetermined dimensions.

For the purpose of the above-described application during themanufacturing process of PolyLED displays, the printing process has tomeet very high requirements. A first requirement is that all pixels needto be printed, as the omission of a pixel will inevitably have anannoying effect on a user of the display, who will be able to perceivethe omission. A second requirement is that the thickness of a certainprinted layer needs to be the same for all individual pixels, as avariation in thickness will result in a variation in light intensity ofthe emitted light over the display. It will be understood that in orderto meet the requirements, the output of the individual pumps of theprint head needs to be the same, and also needs to be constant in time.

In practice, the output of the pumps of the print head changes in time,due to for example clogging which may take place near the outlet of thepump. Therefore, the output of the pumps needs to be checked regularly,resulting in the print head having to be flushed, de-aired or evenreplaced when the output of one or more of the pumps deviates too muchfrom a predetermined output.

It is an objective of the present invention to provide a method forcontrolling the volume of droplets of printing fluid emitted from aprint head, which may be applied to keep the volume of the droplets at aconstant level over time.

The objective is achieved by means of a controlling method whichcomprises the following steps:

-   -   determining a first characteristic frequency of the pump during        a first measurement;    -   actuating the actuator at least one time in order to generate an        actuation pulse which causes at least one droplet of printing        fluid to be emitted from the pump during a first printing        action;    -   determining a second characteristic frequency of the pump during        a second measurement;    -   comparing the second characteristic frequency to the first        characteristic frequency; and    -   determining the value of the actuation pulse which needs to be        generated by the actuator for the purpose of causing at least        one droplet of printing fluid to be emitted from the pump during        a second printing action on the basis of a difference found        between the first characteristic frequency and the second        characteristic frequency, and on the basis of requirements        regarding the volume of the droplet of printing fluid to be        emitted during the second printing action.

According to the present invention, the value of the actuation pulsethat is generated by the actuator is adjusted in time in order to meetthe requirements regarding the volume of the droplet. In this way,changes in the geometry of the pump 10, 20 are compensated for. Therequired order of magnitude of the adjustment of the value of theactuation pulse is determined on the basis of these requirements, on theone hand, and a comparison of characteristic frequencies of the pump, onthe other hand.

For the purpose of manufacturing PolyLED displays, the requirementsregarding the volume of the droplet include keeping the volume of thedroplet constant over time, as is described in the foregoing. In thecase of such requirements being applicable, if the difference foundbetween two subsequently measured characteristic frequencies is zero, itmay be assumed that the value of the actuation pulse does not need to beadjusted in order for the output of the pump during a second printingaction, that will be performed subsequent to the second measurement, tobe the same as the output of the pump during a first printing actionthat is performed between the two measurements. However, if thedifference found between two subsequently measured characteristicfrequencies does not equal zero, the value of the actuation pulse needsto be adjusted in order to assure that the volume of droplets ofprinting fluid emitted during the second printing action is the same asthe volume of droplets of printing fluid emitted during the firstprinting action.

For the purpose of the present invention, knowledge of thecharacteristic frequency of the pump being related to the dimensions ofthe pump is applied in combination with knowledge of the volume of theemitted droplet being mainly determined by the dimensions of the pump.

Using common frequency measurement techniques, determining thecharacteristic frequency of the pump does not require much time. Thedetermination process may be performed so fast, that it is possible toincorporate the determination process in a printing process, withoutinfluencing the speed of the printing process. In such a case, acombined process is obtained, in which the process of controlling thepumps of the print head to fire droplets is alternated with the processof checking the output of the pumps of the print head and determiningthe required adjustment of the value of the actuation pulse that isgenerated by the actuator.

The present invention will now be explained in greater detail withreference to the Figures, in which similar parts are indicated by thesame reference signs, and in which:

FIG. 1 diagrammatically shows a sectional view of a portion of a printhead having Helmholtz-type ink jet pumps;

FIG. 2 diagrammatically shows a single Helmholtz-type ink jet pump;

FIG. 3 diagrammatically shows a sectional view of a portion of a printhead having open-end ink jet pumps;

FIG. 4 diagrammatically shows a single open-end ink jet pump;

FIG. 5 shows a graphical drawing depicting a relation between a meniscusunder-pressure and a measured Helmholtz frequency;

FIG. 6 shows a graphical drawing depicting a relation between dimensionsof a pump, key tone frequency of the pump and speed of sound for aHelmholtz-type ink jet pump;

FIG. 7 shows a graphical drawing depicting a relation between dimensionsof a pump, key tone frequency of the pump and speed of sound for anopen-end ink jet pump;

FIG. 8 diagrammatically shows a system for controlling the actions of aprint head;

FIG. 9 diagrammatically shows a system for measuring a characteristicfrequency of a single pump; and

FIG. 10 diagrammatically shows a system for measuring characteristicfrequencies of a number of pumps.

FIG. 11 shows a flowchart depicting a method for controlling volume ofdroplets of printing fluid.

FIGS. 1-4 show piezo-electrically driven print heads, wherein FIGS. 1and 2 show a portion of a print head 1 having Helmholtz-type ink jetpumps 10, and wherein FIGS. 3 and 4 show a portion of a print head 2having open-end ink jet pumps 20. The print heads 1, 2 may be providedwith one or more rows of inkjet pumps 10, 20.

The pumps 10, 20 comprise a pump chamber 11 for containing printingliquid that will hereinafter also be referred to as ink. At one end ofthe pump chamber 11, a nozzle 12 is provided, which extends between thepump chamber 11 and a nozzle front 13 of the print head 1, 2. At anotherend, the pump chamber 11 is connected to an ink supply channel 14. Thepump chamber 11 of the pumps 10 of the print head 1 as shown in FIGS. 1and 2 are indirectly connected to the ink supply channel 14, through athrottle 15, whereas the pump chamber 11 of the pumps 20 of the printhead 2 as shown in FIGS. 3 and 4 are directly connected to the inksupply channel 14. In view of their designs, the pumps 10 of the printhead 1 as shown in FIGS. 1 and 2 are referred to as Helmholtz-type inkjet pumps 10, whereas the pumps 20 of the print head 2 as shown in FIGS.3 and 4 are referred to as open-end ink jet pumps 20.

The diameter of the nozzle 12 is substantially smaller than the diameterof the pump chamber 11. In the print head 2 as shown in FIGS. 3 and 4,the diameter of the throttle 15 is also substantially smaller than thediameter of the pump chamber 11.

Each individual pump 10, 20 is associated with an actuator 16 comprisinga piezo-electric element, which actuator 16 will therefore hereinafteralso be referred to as piezo-electric actuator 16. At least a portion ofthe wall 17 of the pump chamber 11 is flexible, so that the pump chamber11 contracts when the actuator 16 is actuated and deforms in thedirection of the pump chamber 11.

For the purpose of a printing process, the ink supply channel 14 and thepumps 10, 20 are filled with ink 18. During the printing process, thepumps 10, 20 fire ink droplets in the direction of a carrier (not shownin FIGS. 1-4) like a sheet of paper, a glass substrate or a plasticsubstrate, through the nozzle 12. The ink droplets are generated as aresult of an actuation of the actuator 16, which causes the pump chamber11 to contract. During the contraction of the pump chamber 11, thepressure in the pump 10, 20 is increased, as a result of which a dropletof ink 18 is released through the nozzle 12. The volume of the releaseddroplet is roughly equal to the volume displaced by the actuator 16. Thesize of the droplet and the diameter of the nozzle 12 are related in thesense that the diameter of the droplet is almost equal to the diameterof the nozzle 12.

In order to achieve a high printing quality, the pumps 10, 20 arepositioned at a relatively small pitch. As a consequence, the diameterof the pumps 10, 20 is small relative to the length of the pumps 10, 20,which is relatively large in order to obtain a sufficiently large volumedisplacement.

The speed and the diameter of the droplet are related to each otherthrough a characteristic actuation frequency that is defined by thefollowing equation:

$f_{actuation} = {\frac{{Droplet}\mspace{14mu}{velocity}}{{Rayleigh}\mspace{14mu}{break}\text{-}{up}\mspace{14mu}{length}} = \frac{v_{droplet}}{\pi\; d_{nozzle}}}$wherein:

-   -   f_(actuation) represents the characteristic actuation frequency;    -   v_(droplet) represents the speed of the droplet; and    -   d_(nozzle) represents the diameter of the nozzle 12.

The smaller the diameter of the nozzle 12, the smaller the size of thedroplet, and the higher the actuation frequency needs to be in order toobtain a predetermined value of the speed of the droplet. For thepurpose of obtaining a favourable functioning of the piezo-electricallydriven print heads 1, 2, the actuation frequency should be more or lessequal to the key tone frequency of the pumps 10, 20 of the print heads1, 2. The key tone frequency is related to the design of the print heads1, 2, more in particular the design of the pumps 10, 20.

A characteristic frequency of the pumps 10, 20 containing a fluid columnof ink 18 is the Helmholtz frequency. For the Helmholtz-type ink jetpumps 10, the Helmholtz frequency is given by the following equation:

$f_{Helmholtz} = {\frac{1}{2\pi}\sqrt{\frac{K}{\rho\; A\; L}\left( {\frac{A_{1}}{L_{1}} + \frac{A_{2}}{L_{2}}} \right)}}$wherein:

-   -   f_(Helmholtz) represents the Helmholtz frequency;    -   K represents the compressibility of the ink 18, corrected for        the compliance of the environment;    -   ρ represents the density of the ink 18;    -   A represents the area of the cross-section of the fluid column        in the pump chamber 11;    -   L represents the length of the fluid column in the pump chamber        11;    -   A₁ represents the area of the cross-section of the fluid column        in the nozzle 12;    -   L₁ represents the length of the fluid column in the nozzle 12;    -   A₂ represents the area of the cross-section of the fluid column        in the throttle 15; and    -   L₂ represents the length of the fluid column in the throttle 15.

The compressibility and the density of the ink 18 are related to thespeed of sound, in the following manner:K=ρc ²wherein:

-   -   c represents the speed of sound, corrected for the compliance of        the environment

The length of the throttle 15 is much larger than the length of thenozzle 12, while the cross-sectional dimensions of the throttle 15 andthe nozzle 12 are roughly equal. Therefore, the Helmholtz frequency ismainly dependent on the dimensions of the fluid column in the nozzle 12.

In a situation in which the nozzle 12 is partly clogged, thecross-sectional area A₁ has become smaller. As a consequence, theHelmholtz frequency is lower.

A determining factor in relation to the length of the fluid columncontained in the nozzle 12 is the meniscus under-pressure. When theunder-pressure is too low, the meniscus is retracted in the nozzle 12.As a consequence, the fluid column in the nozzle 12 is shorter and theHelmholtz frequency is higher.

The compressibility of the ink 18 is very sensitive to the presence ofair bubbles in the pump 10, even if these air bubbles are relativelysmall. Air bubbles that are as large as the droplets that need to begenerated are capable of completely blocking the pump 10, as thepressure necessary for forming and firing the droplets cannot be builtup in the pump 10 when such an air bubble is present. The presence ofair bubbles causes the Helmholtz frequency to decrease drastically.

In FIG. 5, a graph is depicted, illustrating a relation between aHelmholtz frequency and the meniscus under-pressure, as obtained by apractical experiment. As already mentioned in the foregoing, the lengthof the fluid column in the nozzle 12 is related to the meniscusunder-pressure.

The graph shows that when the absolute value of a negative pressuredecreases, the Helmholtz frequency decreases as well, as a result of thefact that the length of the fluid column in the nozzle 12 increases.

Further, the graph shows that when the sign of the pressure changes, analmost stepwise drop of the Helmholtz frequency occurs. This is a resultof the fact that the length of the fluid column is abruptly increased,due to wetting of the nozzle front 13.

The experimentally obtained graph proves that the Helmholtz frequency isclosely related to the length of the fluid column in the nozzle 12.Furthermore, the Helmholtz frequency is very sensitive to changes in thelength of the fluid column, which can be derived from the fact that themeasured drop of the Helmholtz frequency is larger than 3,000 Hz.Because of the above reasons, and the fact that frequency measuringtechniques are very accurate, the Helmholtz frequency can very well beused as an indicator of the state of the nozzle 12.

As the length of the fluid column in the pump chamber 11 of the pumps10, 20 is usually large compared to its cross-sectional dimensions, wavepropagation should be taken into account. Due to the occurrence of wavepropagation, a spectrum of resonance frequencies is present, of whichonly the key tone frequency is considered in the following. For theHelmholtz-type ink jet pumps 10, the key tone frequency taking intoaccount wave propagation follows approximately from the followingtranscendental equation:

${\tan\;\frac{\omega\; L}{c}} = {\frac{L}{L_{1}}\frac{A_{1}}{A}\frac{1}{\left( \frac{\omega\; L}{c} \right)}}$wherein:

-   -   ω represents the key tone radial frequency.

FIG. 6 shows a graph which can be used to solve the transcendentalequation tan(x)=C/x. Along the horizontal axis of the graph, the valueof C=LA₁/L₁A is defined.

Along the vertical axis of the graph, the corresponding value of x=ωL/ccan be found, which fulfils the transcendental equation.

When the nozzle 12 is clogged, the area of the cross-section of thefluid column in the nozzle 12 decreases, as a result of which the valueof C decreases. It appears from the graph that as a result, the key tonefrequency decreases as well.

Further, when the meniscus under-pressure is relatively high, the lengthof the fluid column in the nozzle 12 is relatively small. Consequently,the value of C is relatively high and the corresponding value of x isrelatively high, which implies that the key tone frequency is relativelyhigh.

An air bubble present in the pump chamber 11 has an enormous effect onthe compressibility of the ink 18 contained in the pump chamber 11 andleads to a drastic decrease of the speed of sound and the key tonefrequency.

As the key tone frequency is closely related to the dimensions of thefluid column in the Helmholtz-type ink jet pump 10, and is additionallyvery sensitive to air bubbles present in the pump 10, this frequency canvery well be used as an indicator of the state of the pump 10, more inparticular the state of the nozzle 12.

Contrary to the Helmholtz-type ink jet pumps 10, the open-end ink jetpumps 20 do not comprise a throttle 15. For that reason, and the factthat the diameter of the nozzle 12 is much smaller than the diameter ofthe pump chamber 11, the key tone frequency of the open-end ink jetpumps 20 is the so-called λ/4 mode frequency of a tube corrected for thepresence of the nozzle 12. In this way, the following transcendentalequation is obtained:

${\tan\;\frac{\omega\; L}{c}} = {{- \frac{A}{A_{1}}}\frac{L_{1}}{L}\frac{\omega\; L}{c}}$

FIG. 7 shows a graph which can be used to solve the transcendentalequation tan(x)=−Cx. Along the horizontal axis of the graph, the valueof C=AL₁/A₁L is defined. Along the vertical axis of the graph, thecorresponding value of x=ωL/c can be found, which fulfils thetranscendental equation.

When the nozzle 12 is clogged, the area of the cross-section of thefluid column in the nozzle 12 decreases, as a result of which the valueof C increases. It appears from the graph that as a result, the key tonefrequency decreases.

Further, when the meniscus under-pressure is relatively high, the lengthof the fluid column in the nozzle 12 is relatively small. Consequently,the value of C is relatively small and the corresponding value of x isrelatively high, which implies that the key tone frequency is relativelyhigh.

An air bubble present in the pump chamber 11 has an enormous effect onthe compressibility of the ink 18 contained in the pump chamber 11 andleads to a drastic decrease of the speed of sound and the key tonefrequency.

As the key tone frequency is closely related to the dimensions of thefluid column in the open-end ink jet pump 20, and is additionally verysensitive to air bubbles present in the pump 20, this frequency can verywell be used as an indicator of the state of the pump 20, more inparticular the state of the nozzle 12.

Additional to the determination of a characteristic frequency of the inkjet pumps 10, 20 between two printing actions, determination of otherparameters may take place. For example, the pressure rise in the pumps10, 20 during generation of a droplet may be measured. In a pump 10, 20containing an air bubble, the pressure rise is relatively low.Therefore, the pressure rise can be used as an indicator of the presenceof enclosed air in the pumps 10, 20.

In the foregoing, a method according to the present invention forobtaining information regarding the state of ink jet pumps 10, 20 ofprint heads 1, 2, more in particular the state of the nozzle 12 of thepumps 10, 20, is described. This method may advantageously be appliedfor the purpose of controlling print heads 1, 2 that are used in themanufacturing process of PolyLED displays.

PolyLED displays comprise a multitude of rectangular LEDs that areindividually controllable. The LEDs emit light when actuated by means ofan electric current. Each LED comprises a stack of different layers,which are printed on a substrate. The dimensions of the LEDs are verysmall, in order for the human eye not to be able to discern theindividual LEDs of the display. One LED may for example be 200 μm longand 67 μm wide. Suitable values of the thickness of the different layersof the LED are in the nanometre range; the thickness is for example 200nm or even 70 nm. Consequently, the volume of ink droplets containingthe material of a layer needs to be very small. Suitable values of thevolume of the ink droplets are in the picolitre range.

PolyLED displays have many advantages over other types of displays.Contrary to conventional displays, which at the backside comprise alayer of phosphor elements that luminesce when actuated by electronsoriginating from an electron gun, there is no need for PolyLED displaysto be used in combination with additional components being positioned atthe backside of the display and occupying much space. In comparison withLiquid Crystal Displays, the energy consumption of PolyLED displays isrelatively low, and the image is present at every possible viewingangle.

On the basis of the foregoing paragraph, it will be understood thatthere is a great need for reliable techniques for manufacturing PolyLEDdisplays. Ink jet printing processes, which are part of themanufacturing process of PolyLED displays have to meet extremely highstandards. For example, for one of the layers of the LEDs, the so-calledLight Emitting Polymer layer, the thickness of which is 70 nm,variations in the ink dosing should not exceed a value of 2%.Furthermore, non-operation of the ink jet pumps 10, 20 is not allowed,as the PolyLED display must not contain any non-functioning LEDs. Theimportance of meeting the standards is even more evident when the factthat the layer is printed on a pre-patterned carrier that should not bewasted is taken into account.

The above-described method for checking the state of the pumps 10, 20 ofa print head 1, 2, wherein determination of the state takes place on thebasis of measurements of characteristic frequencies of the pumps 10, 20,offers the possibility of accurately controlling the volume of inkdroplets. For example, if the frequency measurements point out that anozzle 12 is somewhat clogged, the actuation pulse may be increased inorder to maintain the predetermined level of droplet volume.

In case a pump 10, 20 contains an air bubble and is not able to performits printing task, the printing process should be interrupted for thepurpose of de-airing the printing head 1, 2.

In order to meet the high standards, during the printing process of aPolyLED display, the state of the pumps 10, 20 of the print head 1, 2 isadvantageously checked every time before an ink droplet is fired. On thebasis of a comparison of a newly measured characteristic frequency witha previously measured frequency, the value of the actuation pulse thatneeds to be generated by the actuator may be accurately determined, orit may appear that the printing process should be stopped and the printhead 1, 2 should be subjected to maintenance or replaced. The previouslymeasured frequency may for example have been determined during a firstmeasurement of a fresh print head 1, 2, which may be a print head thathas just been subjected to maintenance, or which may even be an entirelynew print head 1, 2 which has not been used before.

In FIG. 8, a possible practical system 30 for controlling the actions ofa print head 1, 2 is shown.

The controlling system 30 comprises a computer 31, which is programmedto generate information for controlling the pumps 10, 20 of the printhead 1, 2 on the basis of measured characteristic frequencies of theindividual pumps 10, 20 and requirements regarding the volume of the inkdroplets. The measurements are performed by a measuring device 32, whichis connected to the computer 31.

Further, the controlling system 30 comprises a converting device 33 forconverting the serial information originating from the computer 31 intoparallel information. For the purpose of actually controlling theactions of the individual actuators 16 of the print head 1, 2, acontrolling device 34 is provided. The controlling device 34 is capableof individually controlling the various actuators 16 of the print head1, 2 on the basis of the parallel information as transmitted by theconverting device 33.

Advantageously, use is made of the fact that a piezo-electric elementcan function simultaneously as an actuator and as a sensor. In this way,the characteristic frequency can be measured continuously, so that itcan be assured that every printing action meets the requirements. Acommon four-point measuring technique may be applied, wherein theactuating and sensing actions may be performed at the same time.

It is not necessary to use the entire piezo-electric element as asensor. Instead, the piezo-electric element can be split into twoportions, wherein one portion is used for actuating the pump 10, 20, andwherein another portion is used for measuring the characteristicfrequency of the pump 10, 20.

In FIG. 9, a possible practical system 40 for measuring a characteristicfrequency of a single ink jet pump 10, 20 is shown.

The measuring system 40 comprises an oscillator circuit 41, which isarranged such as to act on the pump 10, 20. The oscillator circuit 41starts to resonate at a suitable frequency, for example the key tonefrequency. The voltage swing of the oscillation is only a few Volts, sothat the pump 10, 20 does not release any ink 18.

The oscillator circuit 41 is constructed such as to output a frequencydependent voltage. An amplifier circuit 42 is provided for amplifyingand buffering the voltage output by the oscillator circuit 41. Further,an analog to digital converter 45 of a suitable resolution is providedfor converting the analogue amplified voltage into a digital output wordthat is representative of the characteristic frequency at which the pump10, 20 is resonating.

In FIG. 10, a possible practical system 50 for measuring acharacteristic frequency of a number of ink jet pumps 10, 20 is shown.

In the shown measuring system 50, each pump 10, 20 is connected to anoscillator circuit 41, and each oscillator circuit 41 is followed by anamplifier circuit 42. All outputs 43 of the amplifier circuits 42 areconnected to a single selection circuit 44.

By applying a digital selection word to the selection circuit 44, theamplified voltage output by one pump 10, 20 is transmitted to an analogto digital converter 45. The converter 45 outputs a digital output wordthat is representative of the characteristic frequency at which the pump10, 20 concerned is resonating.

As stated in the foregoing, when an air bubble gets entrapped in thepump 10, 20, the functioning of the pump 10, 20 is affected to a largeextent. The air bubble may even be large enough to prevent the pump 10,20 from releasing ink 18. Total failure of the pump 10, 20 may also becaused by other factors, for example extreme clogging of the nozzle 12.

In the context of manufacturing PolyLED displays, every time whencomplete failure of a pump 10, 20 occurs, the printing process needs tobe stopped. This is bothersome, as an interruption of the manufacturingprocess costs time and money, but this is necessary in order to meet thehigh standards.

In order to solve the above-sketched problem, according to an importantaspect of the present invention, the print heads 1, 2 comprise at leasttwo rows of pumps 10, 20, wherein the state of the pumps 10, 20 of therows is continuously checked according to the method as described in theforegoing.

If at a certain stage of the printing process, a certain pump 10, 20 isnot able to release ink 18 any more, the measured characteristicfrequency will reveal this state of the pump 10, 20 concerned. In such asituation, the pump 10, 20 at a corresponding position in another rowmay be used to perform the printing action which should actually beperformed by the pump 10, 20 that has fallen out of action. In this way,the time needed for a single printing action may increase, butinterruption of the printing process is prevented. Since there is nocorrelation between the failure mechanisms of the different rows of theprint head 1, 2, it is most unlikely that pumps 10, 20 at correspondingpositions in different rows fail simultaneously or shortly after eachother. Therefore, by having the dosing operation of a non-operating pump10, 20 of a row taken over by another pump 10, 20 of another row anenormous increase in reliability is obtained. It will be understood thatit is important that all areas of a carrier which need to be coveredwith ink 18 can be reached by at least two individual rows of pumps 10,20.

The individual rows of pumps 10, 20 may be controlled such that allpumps 10, 20 are normally involved in the printing process. For example,a pump 10, 20 of a first row may normally fire two droplets of ink 18 inthe direction of a certain area of a carrier, whereas a pump 10, 20 of afollowing row may somewhat later also normally fire two droplets of ink18 in the direction of the same area. In case the pump 10, 20 of thefirst row fails, the following pump 10, 20 is controlled such as to firefour droplets of ink 18 instead of two droplets of ink 18 in thedirection of each area that needs to be covered with ink 18 during theprinting process. It is alternatively possible that the pump 10, 20 ofthe following row fails, and that the pump 10, 20 of the first row iscontrolled such as to fire four droplets of ink 18 in the direction ofeach area that needs to be covered with ink 18 during the printingprocess.

According to another option for controlling the individual rows of pumps10, 20, only the pumps 10, 20 of a first row are normally involved inthe printing process, wherein the pumps 10, 20 of a following row arenot used until the function of at least one pump 10, 20 of the first rowneeds to be taken over.

It will be understood that the same effects as described in theforegoing paragraphs are obtained when two or more print heads 1, 2comprising a single row of pumps 10, 20 are applied. Preferably, in sucha case, the individual print heads 1, 2 follow the same path withrespect to the carrier during the printing process, wherein one printhead 1, 2 follows another print head 1, 2 at a close distance.

Further, it will be understood that it is not necessary to apply tworows of pumps 10, 20 in order for one pump 10, 20 to be able to takeover the function of another pump 10, 20. Even if one single row ofpumps 10, 20 is applied, pumps 10, 20 may take over each other'sfunction when the row is movable in the direction in which it extends.

It is not necessary that the function of a pump 10, 20 that has fallenout of action is taken over by only one other pump 10, 20; it is alsopossible that two or more other pumps 10, 20 are used to ensure that theprinting process can be continued while still meeting the requirements.In the example of the pumps 10, 20 normally firing two ink droplets, thefunction of a pump 10, 20 that has fallen out of action may be performedby two pumps 10, 20, wherein each of the two pumps 10, 20 is controlledsuch as to fire three ink droplets instead of two ink droplets. However,in such a case, both pumps 10, 20 need to be brought to positions wherethe pump 10, 20 that has dropped out would have performed printingactions.

It will be clear to a person skilled in the art that the scope of thepresent invention is not limited to the examples discussed in theforegoing, but that several amendments and modifications thereof arepossible without deviating from the scope of the present invention asdefined in the attached claims.

In the foregoing, a method according to the present invention forobtaining information regarding the state of ink jet pumps 10, 20 ofprint heads 1, 2, more in particular the state of the nozzle 12 of thepumps 10, 20, is described. According to an important aspect of themethod according to the present invention, a characteristic frequency ofthe pumps 10, 20 containing a fluid column of ink 18 is determined. Thecharacteristic frequency provides information concerning the resonancecharacteristics of the pumps 10, 20, which are directly related to thegeometry of the pumps 10, 20. Therefore, determination of thecharacteristic frequency of the pumps 10, 20 offers the possibility ofdetecting changes in the nozzle 12 of the pumps 10, 20.

In case of a change being detected, the consequences of the change aredetermined by its magnitude and requirements regarding the volume of theink droplets to be emitted. When the change in the characteristicfrequency is relatively small and the volume of the ink droplets needsto be maintained at a constant level, the value of the actuation pulsegenerated by the actuator 16 acting on the pump 10, 20 concerned needsto be adjusted. When the change in the characteristic frequency isrelatively large and brings about a decrease of the characteristicfrequency, the conclusion may be that air is present in the pump 10, 20concerned. If that is the case, the function of the pump 10, 20 needs tobe taken over by at least one other pump 10, 20, or the print head 1, 2needs to be flushed and de-aired.

The determined characteristic frequencies may for example be Helmholtzfrequencies or key tone frequencies. Such frequencies can be measured inan accurate and reliable manner, also due to the fact that thesefrequencies are inherent characteristics of the pumps 10, 20 containingink 18, which are not dependent for example on whether the pumps 10, 20are releasing ink 18 or not.

An important advantage of continuously monitoring the characteristicfrequencies of all the pumps 10, 20 of a print head 1, 2 is that theprinting process as performed by the print head 1, 2 can be performed ina very accurate manner. Another advantage is that a well-foundeddecision may be taken regarding replacement of the print head 1, 2.

FIG. 11 shows a flowchart depicting a method for controlling volume ofdroplets of printing fluid according to the present invention. As statedin the foregoing, the present invention provides the method foraccurately controlling the volume of ink droplets emitted from a printhead (1, 2). Specifically, the method for controlling the volume ofdroplets of printing fluid (18) emitted from a print head (1, 2) duringa printing process includes; the print head (1, 2) having: at least onepump (10, 20) having an inlet for taking in the printing fluid (18), apump chamber (11) for containing the printing fluid (18) and an outletfor letting out printing fluid (18); and an actuator (16) for generatingactuation pulses acting on the printing fluid (18) in the pump (10, 20).The method also has the following steps: Step (100): determining a firstcharacteristic frequency of the pump (10, 20) during a firstmeasurement; Step (102): actuating the actuator (16) at least one timein order to generate an actuation pulse which causes at least onedroplet of printing fluid (18 ) to be emitted from the pump (10, 20)during a first printing action; Step (104): determining a secondcharacteristic frequency of the pump (10, 20) during a secondmeasurement; Step (106): comparing the second characteristic frequencyto the first characteristic frequency; and Step (108): determining thevalue of the actuation pulse which needs to be generated by the actuator(16) for the purpose of causing at least one droplet of printing fluid(18) to be emitted from the pump (10, 20) during a second printingaction on the basis of a difference found between the firstcharacteristic frequency and the second characteristic frequency, and onthe basis of requirements regarding the volume of the droplet ofprinting fluid (18) to be emitted during the second printing action.

1. A method for controlling volume of droplets of printing fluid (18)emitted from a print head (1,2) during a printing process; said printhead (1,2) having: at least one pump (10,20) having an inlet for takingin the printing fluid (18), a pump chamber (11) for containing theprinting fluid (18) and an outlet for letting out printing fluid (18);and an actuator (16) for generating actuation pulses acting on theprinting fluid (18) in the pump (10,20); said method comprising thefollowing steps: determining a first characteristic frequency of thepump (10,20) during a first measurement; actuating the actuator (16) atleast one time in order to generate an actuation pulse which causes atleast one droplet of printing fluid (18) to be emitted from the pump(10,20) during a first printing action; determining a secondcharacteristic frequency of the pump (10,20) during a secondmeasurement; comparing the second characteristic frequency to the firstcharacteristic frequency; and determining the value of the actuationpulse which needs to be generated by the actuator (16) for the purposeof causing at least one droplet of printing fluid (18) to be emittedfrom the pump (10,20) during a second printing action on the basis of adifference found between the first characteristic frequency and thesecond characteristic frequency, and on the basis of requirementsregarding the volume of droplets of printing fluid (18) to be emittedduring the second printing action.
 2. The controlling method accordingto claim 1, wherein actuation of the actuator (16) is alternated withdetermination of the characteristic frequency of the pump (10,20)associated with the actuator (16) throughout the printing process. 3.The controlling method according to claim 1, wherein the requirementsregarding the volume of droplets of printing fluid (18) to be emittedduring the second printing action include maintaining a level of thevolume of droplets of printing fluid (18) emitted during the firstprinting action.
 4. The controlling method according to claim 1, whereinthe pump is a Helmholtz-type ink jet pump (10), and wherein each of thefirst and second characteristic frequencies comprises a Helmholtzfrequency or a key tone frequency of the pump (10).
 5. The controllingmethod according to claim 1, wherein the pump is an open-end ink jetpump (20), and wherein each of the first and second characteristicfrequencies comprises a key tone frequency of the pump (20).
 6. Thecontrolling method according to claim 1, wherein the first measurementis performed on a fresh print head (1,2).
 7. The controlling methodaccording to claim 1, wherein the actuator (16) comprises apiezo-electric element, wherein the piezo-electric element is used as afrequency sensor for determining the characteristic frequency of thepump (10,20).
 8. The controlling method according to claim 1, whereinthe printing process as performed by the pump (10,20) is stopped in caseit appears that the determined value of the actuation pulse which needsto be generated by the actuator (16) for the purpose of causing at leastone droplet of printing fluid (18) to be emitted from the pump (10,20)during a second printing action cannot be set.
 9. The controlling methodaccording to claim 8, wherein the print head (1,2) comprises at leasttwo pumps (10,20), and wherein at least one of the pumps (10, 20) iscontrolled to take over the function of another of the pumps (10,20) incase the printing process as performed by the latter has been stopped.